1. Field of the Invention
The present invention relates to resin gears and manufacturing methods thereof.
2. Description of the Related Art
For example, in electric power steering systems, rotation of an electric motor for steering assist is reduced in speed and amplified in output via a speed reducer and is then transmitted to a steering operation mechanism, thereby assisting operation of the steering operation mechanism which is performed by operation of the driver. A speed reducer including a worm and a worm wheel which mesh with each other is usually used as such speed reducer. The worm wheel is typically manufactured by forming an annular resin member around a sleeve made of, e.g., iron by injection molding (insert molding) etc. and then forming teeth on the outer periphery of the resin member by cutting etc.
For example, the resin member is made of a polyamide (PA6, PA66, PA46, etc.), an aromatic polyamide, a polyacetal, PEEK, PPS, etc. Further reduction in weight of automotive parts is desired in response to the recent need for reduced environmental load. The weight of the iron sleeve of the worm wheel is relatively great in the overall weight of the electric power steering system. It is therefore necessary to reduce the overall weight of the worm wheel including such a core while maintaining required strength and rigidity.
In recent years, fiber reinforced composites that are lightweight, strong, and rigid have been increasingly applied to automotive parts. Examples of the fiber reinforced composites include carbon fiber reinforced plastics (CFRP) using carbon fibers as reinforcing fibers and using thermosetting resins, and carbon fiber reinforced thermoplastics (CFRTP) using carbon fibers as reinforcing fibers and using thermoplastic resins.
For example, the following methods are possible in order to reduce the weight of gears of worm wheels etc. by using such fiber reinforced composites.                (1) A prepreg, which is a reinforcing fiber sheet impregnated with a thermosetting resin, is wound in an annular shape. Then, a disc-shaped molding made of the fiber reinforced composite and having a shape corresponding to the overall shape of the gear is produced by sheet winding in which a thermosetting resin is cured. Teeth are formed in the outer periphery of the disc-shaped molding by cutting etc.        (2) A sleeve made of a fiber reinforced composite is produced by the sheet winding. Then, an annular resin member is formed on the outer periphery of the sleeve by insert molding etc. in a manner similar to that of conventional examples. Teeth are formed in the outer periphery of the resin member by cutting etc. (Japanese Patent Application Publication No. 2001-304379 (JP 2001-304379 A).        
Since the gear formed by the method (1) is made of the fiber reinforced composite up to the tip ends of the teeth, this gear is rigid but is not very tough. For example, in the case of the worm wheels, noise called rattling noise (tooth knocking noise) tends to be generated when the gear meshing with the worm is rotated. Moreover, continuous reinforcing fibers are cut or the resin is separated from reinforcing fibers when forming the teeth by cutting. This can be a fracture origin, thereby reducing shock resistance and mechanical strength of the gear.
The gear formed by the method (2) is not satisfactory in terms of reliability of bonding between the sleeve and the tooth portion, thermal shock resistance, etc. If knurling, blasting, etching, etc. is performed on the outer periphery of the sleeve which is in contact with the tooth portion in order to improve these characteristics and to retain the tooth portion on the sleeve, to prevent relative rotation between the tooth portion and the sleeve, etc., continuous reinforcing fibers are cut or the resin is separated from reinforcing fibers. This can be a fracture origin, thereby reducing shock resistance and mechanical strength of the gear.
The resin that is used for the fiber reinforced composite in the methods (1) and (2) is limited to flexible epoxy resins having appropriate stickiness before being cured, etc. However, in the case of fiber reinforced composites using such epoxy resins etc., it takes as long as about five hours or more (including time required for fluidization) until the curing reaction is completed. This significantly reduces productivity of gears and electric power steering systems including the gears.